Hydrogen is used as a feedstock for many chemical processes and has been proposed as an alternative fuel especially for use in fuel cells in stationary and mobile facilities. Steam reforming of hydrocarbon-containing feedstock is a conventional source of hydrogen. Steam reforming of hydrocarbons is practiced in large-scale processes, often at a facility having refinery or chemical operations. Thus, for instance, the large-scale hydrogen plant will likely be able to draw upon the skills within the entire facility to operate sophisticated unit operations to enhance hydrogen production efficiency. An additional benefit of having a large scale hydrogen plant within a facility having refinery or chemical operations is that the steam generated in the hydrogen plant from cooling the steam reforming effluent and by heat exchange with the combustion of waste gases has value to such other refinery or chemical operations. The benefits of practicing steam reforming in large-scale plants are also apparent from the nature of the equipment and process. For instance, steam reforming generally uses very high temperatures, often in excess of 800° C., which in turn requires expensive materials of construction. Furthermore, large-scale hydrogen plants typically provide hydrogen product purity in excess of 99 volume percent with less than 10 parts per million by volume (ppmv) of carbon monoxide.
While the economics of large-scale steam reforming make attractive the shipping of hydrogen from such a large-scale reformer to the point of use, hydrogen, nevertheless, is difficult to store and distribute and has a low volumetric energy density compared to fuels such as gasoline. Thus an interest exists in developing economically and practically viable smaller-scale hydrogen generators to provide hydrogen from a hydrocarbon-containing feedstock for use or distribution at a point proximate to the consumer.
There are a number of practical hurdles for such a smaller-scale hydrogen generator to overcome before it is commercially viable beyond overcoming the loss of economy of scale. For instance, the smaller scale may not make economically viable retaining sophisticated operating and technical staff and thus the hydrogen generator must be able to operate reliably with minimal operator support while still providing an economically acceptable hydrogen product meeting purity specifications.
Often smaller-scale hydrogen generators face problems that do not occur with large-scale hydrogen plants. An example is that the demand for hydrogen product or purity may change from time to time. Additionally, the source of hydrocarbon-containing feedstock may change due to availability or cost. Moreover, whether or not a given hydrogen generator will need to accommodate a change in feedstock, manufacturers of smaller-scale hydrogen generators would benefit from having a generator that can accommodate the feedstock sought by the customer. And, smaller-scale hydrogen generators may be stand alone units with no chemical or refinery operation for integration to improve combined economics.
Due to capital expense and a more facile ability to turn down production rates, alternative reforming technology such as partial oxidation/steam reforming, including autothermal reforming, has been considered instead of steam reforming. But as a portion of the feed is oxidized in the reformer, efficiency penalties are taken that are not incurred by steam reforming.
Doshi, et al., in WO 2005/118126, published Dec. 15, 2005, disclose hydrogen generators and processes for generating hydrogen using partial oxidation/steam reforming, especially autothermal reforming, that can achieve attractive efficiencies while still taking advantage of the lower capital costs. These processes have conversion efficiencies (Net Hydrogen Efficiencies or NHE) of at least about 50 percent, preferably at least about 55 percent, without a water gas shift. With a water gas shift, net hydrogen efficiencies of at least about 55, and often in excess of 60, percent may be achieved. The Net Hydrogen Efficiency is the ratio of lower heating values of the recovered hydrogen product stream to the lower heating value of the hydrocarbon feed stream:
      N    ⁢                  ⁢    H    ⁢                  ⁢    E    =                    P        ×        L        ⁢                                  ⁢        H        ⁢                                  ⁢                  V          P                            F        ×        L        ⁢                                  ⁢        H        ⁢                                  ⁢                  V          F                      ×    100  
where P=molar flow of net hydrogen product (mol/hr)                LHVP=lower heating value of product hydrogen (kJ/mol)        F=molar flow of hydrocarbon feedstock (mol/hr)        LHVF=lower heating value of hydrocarbon feedstock (kJ/mol).        
The processes disclosed by Doshi, et al., effect the partial oxidation/steam reforming at high pressures, e.g., at least about 400, preferably at least about 500, kPa absolute. They disclose that the undue adverse effect from high pressure reforming is avoided by the use of a heat integrated steam cycle employing a ratio of steam to carbon in the hydrocarbon-containing feedstock above about 4:1. Doshi, et al, teach the use of a heat integrated steam cycle to counter the adverse effect of pressure and of energy consumption required to vaporize the higher amounts of steam. The heat integrated steam cycle takes advantage of the increased mass of effluent from the partial oxidation reformer to generate at least about 40, and preferably at least about 50, percent of the steam for supply to the reformer at a high temperature, e.g., at least about 300° C. or 350° C., preferably at least about 400° C., say 450° to 600° C.
In the preferred aspects of their invention, Doshi, et al., disclose taking advantage of waste gas from hydrogen purification operations such as membrane separations and pressure swing adsorptions. The waste gas is combusted to generate, in combination with the steam generated by cooling the effluent from the reformer, at least about 90 percent of the steam supplied to the reformer. The heat from the combustion is also used to heat at least a portion of the feed to the partial oxidation reformer. In these preferred aspects, steam and heat are obtained from the unrecovered hydrogen instead of consuming additional hydrocarbon-containing feedstock.
Although Doshi, et al., have made significant advances in providing efficient hydrogen generators that take advantage of low capital costs, additional benefits can be realized in enhancing the flexibility of the hydrogen generator especially by being able to use a wide variety of fuels, including normally liquid and normally gaseous fuels; by being able to achieve a high turndown ratio; and by being able to produce a hydrogen product within a wide range of hydrogen purities, all without unduly adversely affecting the capital costs and Net Hydrogen Efficiency of the hydrogen generator.
Achieving these additional benefits is not without problems. For instance, since the feed to the partial oxidation/steam reformer reactor contains oxygen as well as the hydrocarbon-containing feedstock, care must be taken to assure that adverse effects such as pre-combustion are avoided. Additionally, with liquid fuels, the fuels must be vaporized such that a uniform mixture of hydrocarbon-containing feedstock, steam and oxygen are passed through the reformer. Also hydrogen purity of the product has a significant effect on the amount of unrecovered hydrogen available for combustion to provide heat to one or more streams to the reformer. With higher purity hydrogen products, especially with pressure swing sorption purification, the portion of hydrogen that is contained in the purge and thus unrecovered, is greater than that where a lower hydrogen purity product is sought. Achieving the ability to operate over wide turndown ranges can additionally pose problems especially in an energy integrated hydrogen generator.